hdkeychain: Consolidate tests into package.

[davecgh]

Putting the test code in the same package makes it easier for forks since they don't have to change the import paths as much.

Also, address a few style and consistent nits while here:

• Prefer t.Fatalf over t.Errorf followed by a return
• Use the consistent style of starting a test function comments with the test name
• Prefix test errors by the function being called instead of the one doing the calling since the caller itself is already logged by the test framework
• Check err in max depth test before checking the returned key is nil